2017B Question 06
Describe the effects of morbid obesity on the respiratory system.
Examiner Report
52.2% of candidates achieved a pass in this question.
This question required a broad consideration of the physiological and anatomical parameters which change in the respiratory system of the morbidly obese. Candidates who scored well systematically outlined the various domains of the respiratory system, listed the changes seen and went on to describe the consequences and inter-relationship of these changes. In addition there were some marks allocated to the pathophysiological changes seen - as obesity is itself a pathology - however a sole focus on pathology was insufficient to achieve a pass. Notably there were no marks achieved for describing the metabolic, endocrine or cardiovascular effects of morbid obesity.
Model Answer
Structure:
- Introduction
- Anatomy
- Lung volumes
- Mechanics
- Resistance and compliance
- Ventilation and perfusion
- Gas exchange
- Disease: OSA, OHS
Introduction
| Factor | Detail |
|---|---|
| Definition | - BMI >35kg/m2 |
| Overall effect | - ↑ Resp demand - ↓ Resp reserve |
| Pathophysiology | 1. Mass effect (ME) - Compression of neck, chest - Displacement of diaphragm - Worse if male (↑ central and visceral fat) - Better if female (↑ peripheral and subcut fat) 2. ↑ Metabolic rate (↑ MR) - Due to ↑ muscle (~20% of extra mass) > ↑ fat (~80% of extra mass) 3. Adipokines from visceral fat (AK) - Mostly pro-inflammatory: Leptin, TNFα, IL6, resistin, angiotensinogen, PAI-1 - Some anti-inflammatory: Adiponectin |
Anatomy
| Airway region | Detail |
|---|---|
| Upper airway | - ME → ↓ Radius → ↑ Resistance - ME → ↓ ROM head and neck: - ↑ Risk OSA, OHS, obstruction when sedated - ↑ Risk difficult bag/mask ventilation and intubation |
| Lower airway | - ME → ↓ Lung volume → ↓ Airway radius → ↑ Resistance - AK → Airway inflammation → ↑ Resistance: - ↑ Airway pressure - ↑ Risk gas trapping |
Lung Volumes
| Compliance | Detail |
|---|---|
| Static | - ME → ↓ Chest wall compliance → ↓ TLC, RV, FRC (e.g. ↓ 25% if erect at BMI 30) - Worse if supine, under GA - If FRC < closing capacity: Small airway closure → ↑ Shunt: - ± Supine hypoxaemia - Rapid desaturation after induction |
| Dynamic | - ME → ↓ Chest wall compliance → ↓ FVC - ME → ↑ Resistance → ↓ FEV1 - ↑ MR → ↑ VT 20% |
Breathing Mechanics
| Impediment | Detail |
|---|---|
| Restriction | - ME → ↓ Chest wall compliance: - ↑ Work of breathing (WOB) - Severely impaired ventilation if Trendelenburg |
| Obstruction | - ME → ↓ Airway radius - AK → Airway inflammation: - ↑ WOB |
Alveolar Time Constants
| Factor | Detail |
|---|---|
| Definition | |
| Resistance | - ↑ R due to ME, AK as above |
| Compliance | - ↓ C: Due to ME → ↓ Lung volume → ↓ Alveolar radius ( |
| T | - ↑ Variability: - ↑ Slope phase 3 capnogram - ↑ Peak-plateau pressure difference |
Ventilation and Perfusion
| Factor | Detail |
|---|---|
| V | - ↑ MR → ↑ RR 40%, ↑ TV 25% - Accessory muscle use at rest: - ↑ WOB further - ↓ Reserve if unwell e.g. Pneumonia, exercise |
| Q | - ↑ MR → ↑ Cardiac output - ± Risk OSA/OHS → Chronic ↓ PaO2 → +/- pulmonary hypertension: - Risk RV failure peri-op (e.g. Spont vent sedation → ↑ PaCO2 → ↑↑ PA pressure) |
Gas Exchange
| Factor | Detail |
|---|---|
| ↓ V/Q matching | - FRC below closing capacity → Shunt |
| ABG changes | - ↓ PaO2 (if shunt) - ↑ PaCO2 (if OHS) - ↑ HCO3- (can be very high if OHS) - pH usually normal if chronic process |
Disease
| Disease | Detail |
|---|---|
| OSA | - ME + sleep → Airway collapse → ↓ PaO2 → ↑ SNS activity → Arousal → Repeat: - ↓ Sensitivity to ↓ PaO2 - ↑ Risk obstruction and apnoea peri-op - ↑↑ Risk with opioids, benzodiazepines - 15% have pulmonary hypertension |
| OHS | - ME → ↑ WOB → ↓ VA → Chronic ↑ PaCO2: - ↓ Sensitivity to ↑ PaCO2 - Reliant on hypoxic stimulus (ablated by volatile anaesthetics) - 50% have pulmonary hypertension |